1. Field of the Invention
The present invention relates to a liquid crystal display device (hereinafter, referred to as an LCD device) used as a display device for a television, a word processor, a notebook computer or the like; and a method for driving the same.
2. Description of the Related Art
One known LCD device is an active-matrix type LCD device including thin-film transistors (hereinafter, referred to as TFTs) functioning as switching elements provided in a matrix on an insulation plate made of glass or the like. The active-matrix type LCD device is expected to realize a flat panel display with high quality.
FIG. 8 is a cross-sectional view showing an exemplary configuration of a conventional LCD device. FIG. 9A is a plan view showing an exemplary configuration of one picture element area of an active-matrix substrate in the conventional LCD device shown in FIG. 8. FIG. 9B is a cross-sectional view of the active-matrix substrate taken along line C-C' in FIG. 9A.
As shown in FIG. 8, the conventional LCD device 800 includes the active-matrix substrate 800a including TFTs 3, drain electrodes 4 (display electrodes, only one drain electrode 4 is shown in FIG. 8), gate lines (scanning lines, FIG. 9A), and data lines (signal lines, FIG. 9A) provided on an insulation plate 6a; and a counter substrate 800b including a counter electrode 5, a color filter 13, and a light-shielding film 14 provided on an insulation plate 6b. The active-matrix substrate and the counter substrate 800b are attached to each other with a liquid crystal layer 15 interposed therebetween.
As shown in FIGS. 9A and 9B, the active-matrix substrate 800a of the LCD device includes the TFTs 3 functioning as switching elements provided in a matrix on the insulation plate 6a. A gate 1c of each TFT 3 is a part of the gate line 1. The TFT 3 is driven by a signal input into the gate 1c of the TFT 3. A source 2c of the TFT 3 is branching from the data line 2. A video signal (a display signal) is input from the source 2c of the TFT 3. Each of the gate lines 1 and each of the data lines 2 are provided so as to cross each other. A drain 806 of the TFT 3 is branching from the drain electrode 4. The drain electrode 4 and a storage capacitor line 8 face each other with a gate insulation film 7 (FIG. 9B) interposed therebetween. At the portion where the drain electrode 4, the gate insulation film 7 and the storage capacitor line 8 overlap, a storage capacitor is formed.
In the thus configured LCD device, the drain electrode 4 in the active-matrix substrate 800a and the counter electrode 5 in the counter substrate 800b are driven so as to apply an electric field to the liquid crystal layer 15 in a direction perpendicular to the surfaces of the insulation plates 6a and 6b (FIG. 8). Hereinafter, an LCD device in which an electric field is applied to the liquid crystal layer in a direction perpendicular to the surfaces of the insulation plates 6a and 6b (FIG. 8) will be referred to as an LCD device with a vertical electric field driving system.
An LCD device generally has a problem of a narrow range of viewing angles. For example, inversion and reduction in the contrast ratio are caused in accordance with the angle at which a viewer watches the display screen (viewing angle). Such phenomena are caused because the angle between the light transmitted through the liquid crystal layer and the principal axis of the liquid crystal molecules varies in accordance with the viewing direction, resulting in birefringence anisotropy of the liquid crystal material.
In the above-described LCD device with the vertical electric field driving system, for example as shown in FIG. 8, an electric field E1 is applied to the liquid crystal layer 15 in a direction perpendicular to the surfaces of the insulation plates 6a and 6b, thereby orienting the principal axis of liquid crystal molecules 16 along a direction perpendicular to the insulation plates 6a and 6b. In this manner, light transmittance is controlled. Thus, the angle between the light transmitted through the liquid crystal layer and the principal axis of the liquid crystal molecules significantly varies in accordance with the viewing direction. As a result, in the LCD device with the vertical electric field driving system, the contrast ratio greatly varies in accordance with the viewing direction, thus narrowing the range of viewing angles.
In order to realize a wider range of viewing angles by preventing the inversion and the reduction in the contrast ratio, various attempts have been made. For example, Japanese Laid-Open Patent Publication No. 7-36058 discloses an LCD device in which an electric field is applied to the liquid crystal layer in a direction parallel to the surfaces of the substrates. Hereinafter, an LCD device in which an electric field is applied to the liquid crystal layer in a direction parallel to the substrates will be referred to as an LCD device with a horizontal electric field driving system.
In the LCD device with the horizontal electric field driving system, an electric field is applied to the liquid crystal layer in a direction parallel to the surfaces of the insulation plates 6a and 6b, thus rotating liquid crystal molecules in a plane parallel to the surfaces of the insulation plates 6a and 6b. In this manner, light transmittance is controlled. Accordingly, the angle between the light transmitted through the liquid crystal layer and the principal axis of the liquid crystal molecules is maintained regardless of the viewing direction. As a result, according to the LCD device with the horizontal electric field driving system, the contrast ratio is maintained regardless of the viewing direction, thus realizing a wider range of viewing angles. However, the LCD device with the horizontal electric field driving system has a complicated arrangement of lines as compared with the LCD device with the vertical electric field driving system, thus resulting in a low aperture ratio.
As described above, the LCD device with the vertical electric field driving system has a simple structure and a high aperture ratio, but has a disadvantage in the narrow range of viewing angles. On the other hand, the LCD device with the horizontal electric field driving system has a wider range of viewing angles, but has disadvantages in the complicated structure and the low aperture ratio.
As to viewing angles of LCD devices, a wider range of viewing angles is generally considered to be preferable. However, in the case where a notebook computer is used in an airplane or in a train, the user sometimes does not want others to see the display screen. In this case, a narrow range of viewing angles is more preferable.
However, in the conventional LCD devices, a range of viewing angles is determined by the structure and the kinds of liquid crystal materials used. Therefore, it is impossible for a user to change the viewing angle characteristics of a conventional LCD device in accordance with a desired use and the specific conditions encountered with such a use.